I. Retroviruses
The Retroviridae virus family encompasses all viruses containing an RNA genome and producing an RNA-dependent DNA polymerase (reverse transcriptase). The family is divided into three subfamilies: (1) Oncovirinae, including all the oncogenic retroviruses (referred to in the older literature as "oncornaviruses") and several closely related nononcogenic viruses (collectively referred to herein as "oncoretroviruses"); (2) Lentivirinae, the "slow retroviruses," such as the human immunodeficiency virus (HIV) and visna virus; and (3) Spumavirinae, the "foamy" retroviruses that induce persistent infections, generally without causing any clinical disease. The retroviruses may also be classified based upon virion morphology as type B, type C, or type D retroviruses (Weiss et al., (eds), RNA Tumor Viruses, Cold Spring Harbor Laboratory, NY, 1985, Vol. 1, pp.31-34, 46-51, and Vol. 2, pp. 2-7). More detailed descriptions of the retroviruses may be found in Weiss et al., 1985, Volumes 1 and 2; Doolittle, et al., Quart. Rev. Biol., 64:1-29, 1979; and Varmus and Brown, Retroviruses, p. 53-108, in Berg and Howe, (eds), 1989, Mobile DNA, American Society for Microbiology, Washington, DC.
In broadest overview, the life cycle of a retrovirus comprises entry of an infectious retroviral particle into a host cell, integration of the virus' genetic information into the host cell's genome, and production of new infectious retroviral particles by the biosynthetic machinery of the infected host cell. More specifically, upon entering a cell, a retroviral particle initiates a series of interactive biochemical steps that result in the production of a DNA copy of the virus' RNA genome and its integration into the nuclear DNA of the cell. This integrated DNA copy is referred to as a provirus and can be inherited by any daughter cells of the infected cell like any other gene. Genes contained within the integrated provirus may be expressed in the host cell.
All retroviral particles share common morphological, biochemical, and physical properties, including:
(1) A linear, positive-sense, single-stranded RNA genome composed of two identical subunits and making up about 1% of the mass of the virus. PA0 (2) At least three types of proteins encoded by the viral genome, i.e., gag proteins (the group antigen internal structural proteins), pol proteins (the RNA-dependent DNA polymerase and integrase proteins), and env proteins (the viral envelope protein or proteins). These proteins together make up about 60%-70% of the mass of the virus. PA0 (3) Lipid derived from the cell membrane of an infected cell making up about 30%-40% of the mass of the virus. PA0 (4) Carbohydrate associated with the env proteins, making up about 2-4% of the mass of the virus. PA0 (5) An overall spherical morphology with variable surface projections. PA0 (6) An isocahedral capsid structure containing a ribonucleoprotein complex within an internal nucleoid or nucleocapsid shell.
In addition to genes encoding the gag, pol, and env proteins, the genome of the retrovirus includes two long terminal repeat (LTR) sequences, one at each end of the linear genome. These 5' and 3' LTRs serve to promote transcription and polyadenylation of viral mRNAs. Adjacent to the 5' LTR are sequences necessary for reverse transcription of the viral genome (the tRNA primer binding site) and for efficient encapsulation of viral RNA into particles (the Psi site). Other genes may also be found between the 5' and 3' LTRs of the retroviral genome.
If heterologous genes are inserted in between the 5' and 3' LTRs of a retroviral genome, which is then packaged into a functional retroviral particle, the resulting recombinant retroviral particle is capable of carrying the heterologous genes into a host cell. Upon integration of the recombinant retroviral genome into the host cell's genome as part of the proviral DNA, the heterologous genes may be expressed.
These properties and capabilities have led to the development of retroviral vectors, retroviral packaging and producer cells, and retroviral vector particles (collectively referred to as retroviral transduction systems) as efficient means of stably introducing exogenous genes of interest into mammalian cells. Certain retroviruses have been engineered to produce non-infectious retroviral transduction systems that are especially useful in the field of gene therapy. See Anderson, 1992; Miller, 1992; Mulligan, 1983; Mann, 1983; Cone and Mulligan, 1984.